Refrigerant compressors of the double-acting swash plate type having compression chambers on both sides of each double-acting piston are known which can vary delivery capacity by use of a control device which is designed to render the compression chambers on side under "loaded" and "unloaded" (or idle) conditions alternately in response to the varying cooling capacity demand, thus making possible the change of the compressor's delivery capacity without depending on a clutching operation. For this purpose, two methods have been proposed heretofor. According to one method disclosed by Publication of Japanese Patent Application No. 59-200084 (1984), discharge-suction pressure differential is utilized for controlling of the load-unload changing, which method may be referred to as internal control system. The other method disclosed by U.S. Pat. No. 4,403,921 makes use of electrical control signals provided by any external device adapted for detecting the varying cooling load, which method may be referred to as external control system.
For the sake of better understanding of the background on which the present invention is based, reference is made to FIGS. 9 and 10 of the attached drawings which show compressors operable according to these internal and external control systems, respectively.
In the compressor having an internal control system shown in FIG. 9, a movable, pressure responsive plunger 24 having a discharge valve 14R and retainer 26 screwed to its front side and a variable volume chamber 25 formed on the opposite back side thereof is slidably mounted in rear housing 15R substantially in alignment with a drive shaft 2 mounted centrally in the compressor. There is provided a by-pass port 21 bored through a rear valve plate 10R for communication with a crankcase 4, in which a swash plate drive mechanism is incorporated, through a by-pass passage 22' formed in a spring holder 22 and space formed over the rear end of the drive shaft 2. A spring 28 is mounted in the spring holder 22 for urging the plunger 24 in rearward direction which causes the discharge valve 14R to move away from the discharge port 12R. On the other hand, the variable volume chamber 25 behind the plunger 24 communicates by way of a connection passage 25' with a front discharge chamber 18F. In operation, the plunger 24 is actuated to and fro in response to the pressure differential between discharge pressure applied to the back side of the plunger and suction pressure acting on the front side thereof, whereby the compression chambers on the rear side of the compressor are "unloaded" when the plunger 24 is moved to where the discharge valve 14R is kept away from the discharge port 12R and the by-pass port 21 is opened to thereby allow the refrigerant gas displaced by pistons 5 to be by-passed into the crankcase 4.
Then referring to FIG. 10 illustrating a compressor whose delivery capacity is externally controlled, the compressor per se is substantially the same as that shown in FIG. 9, except that the chamber 25 behind the plunger 24 is selectively communicable with a suction chamber (rear 17R or front 17F) and front discharge chamber 18F through controlled operation of respective solenoid valves 62 disposed between a discharge flange and a suction flange and actuated by control signals supplied from a sensor provided in the suction conduit line for detecting the varying temperature or cooling load through a control 64 (or amplifier) operable on the valves 62 in response to the control signals. Depending on the actuation of the valves 62, suction and discharge pressures are applied selectively to the rear side of the plunger 24, thereby loading and unloading the compression chambers on the rear side of the compressor in the same manner as in the internal control system.
With the compressor having the internal control system, however, the pressure differential between the discharge pressure applied to the back side of the plunger 24 and the suction pressure acting on the opposite front side is varied in response not only to the cooling load, but also to other irrelevant factors such as rapid changes in engine or compressor speed. As a result, the compressor with the internal control system has offered disadvantages in that the loading/unloading control tends to be performed erroneously and the discharge valve 14R attached to the pressure responsive plunger 24 is moved too sensitively, causing too frequent repetition between opening and closing operations and hence inaccurate operation of the discharge valve with respect to the varying cooling load. In addition, such frequent repetition of the discharge valve operation affects its durability and causes poor sealing between the discharge valve 14R and the valve plate 10R, allowing high-pressure, high-temperature discharge gas to enter the low-pressure suction chamber, with the result that the bearings, shaft seals, etc. are deteriorated by such discharge gas. With the external control system shown in FIG. 10, these problems may be solved, but it requires costly external parts including a temperature sensor, control for processing control signals transmitted by the sensor, and solenoid valves for controlling refrigerant gas flow. The use of such additional parts only makes the overall system more complicated and hence more costly to manufacture.